The invention relates to a positive displacement fluid flow proportioning device.
Fluid flow proportioning devices are required, for example, where a two-part mixture of materials is to be used for spraying a coating on metal sheets or within a metal pipeline. In such cases, the materials concerned may well be a cold setting resin material mixed with a liquid curing agent or initiator and, if so, it will be vital that the materials are mixed together in precisely the correct proportions. However, variations of temperature (and thus the viscosities of the materials concerned) as the operation proceeds can make the maintenance of the correct proportions in this particular situation very difficult indeed. In other industrial situations, for example in the manufacture of foodstuffs and pharmaceuticals, where liquids or gases are to be mixed together in exact proportions, other variables can make the maintenance of the ideal mix equally difficult.
A further difficulty in this and other situations where the materials to be mixed are particularly aggressive, for example of an abrasive nature, is that any resilient sealing means required are prone to severe wear, so much so that their frequent replacement to ensure the proper functioning of the proportioning device can make the operation uneconomic.
The object of the invention is to provide a simple fixed-ratio and relatively inexpensive fluid flow proportioning device whereby fluids can be divided accurately into two or more flows which are in some required fixed proportion to each other.
According to the invention, there is provided a fluid flow proportioning device including at least three spool type valve elements slidably located in respective cylindrical bores to which there is connected a common inlet passage for a first fluid flow under pressure, by way of respective inlet ports, a common inlet passage for a second fluid flow under pressure, by way of other respective inlet ports, respective outlet ports being spaced from the inlet ports and respective transfer ports being provided whereby, in response to sequential movements of the valve elements, one of a pair of pressure chambers at the opposite ends of each valve element is fed in turn with one of the two fluid flows under pressure admitted via one of the two transfer ports of the previously actuated element whilst a measured volume of the other fluid flow is discharged, by the movement of the respective valve element, from the other of the pair of pressure chambers via the other transfer port of the previously actuated element, whereby, as the valve elements are actuated in turn, the proportions of the two fluid flows being delivered for use are dependent on the number of pressure chambers for each fluid flow which are connected for use, the respective cylindrical bores in which the valve elements are slidably located being formed in oppositely disposed and spaced apart valve block halves, and central portions of the spool type valve elements being of relatively small diameter, that is to say smaller than the diameters of the cylindrical bores in which the spool type valve elememts are slidably located, and being provided with resilient seals where they extend outwards from the valve block halves. Each spool type valve element may be constituted by two opposite end portions and a central push-rod portion abutting together end to end. Means may be provided for intensifying pressure on the resilient seals which surround the central portions of the spool type valve elements. Provision may be made for any leakage of fluid past a valve element in a cylindrical bore of a valve block half to be ducted back to a respective holding tank for the fluid concerned. Respective baffle plates may be mounted on the central portions of said valve elements to prevent the mixing together of any leakage of fluids past the resilient seals where such mixing could be either dangerous or inconvenient.
The arrangement may be such that, in operation of the device, the pressure of one fluid flow causes movements of the spool type valve elements, in turn, in one direction and the pressure of the other fluid flow then causes movements of said elements, in turn, in the opposite direction. Alternatively, the arrangement may be such that the pressures of the two fluid flows are effective alternately, that is to say, when the pressure of one fluid flow has caused the movement of one valve element in one direction, the pressure of the other fluid flow then causes the movement of a next valve element in the opposite direction, and so on, this resulting in a smoother fluid output from the device.
The first and second fluid flows may be of different fluids and may be at different pressures, the device operating to control the flow of the respective fluids in some fixed proportion to each other, possibly for mixing together at some fixed ratio.
Alternatively, the first and second fluid flows may be taken from a single fluid supply, the device operating to divide the fluid into at least two flows in some fixed proportion to each other. If the device is to control the flow of two different fluids at different pressures, the opposite ends of the valve elements may be of different diameters to compensate for the differences of pressure; to compensate for such different diameters, that is to say to ensure that the same volume of fluid is displaced on each movement of a valve element, each valve element may be constituted by a pair of valve halves arranged to move through different distances, that is to say, being linked together for simultaneous movement by means which control the stroke of each valve half in inverse proportion to its cross-sectional area.